The words "affect" and "effect" are often confused due to their similar sounds. Generally, "affect" is a verb meaning "to influence" or "to produce a change in," while "effect" is typically a noun meaning "a result" or "an outcome." "Effect" can also be a verb meaning "to bring about" or "to cause to happen," but this usage is less common.
Option D, "The effect of the speech was powerful," is correct. Here, "effect" is used correctly as a noun, referring to the result or impact of the speech.
Option A is incorrect because "affect" is used as a noun ("an immediate affect"), but the context requires the noun "effect." Option B is incorrect because while "effect" can be a verb meaning "to bring about," the more common and natural phrasing for "positive change" would be "will bring about positive change" or "will have a positive effect." "Effect positive change" is grammatically awkward. Option C is incorrect because "effect" is used as a verb where "affect" (to influence) is needed: "The weather can affect your mood."